The present invention relates to a method for obtaining a composite material part incorporating a tensioning step of the filaments, as well as a tool for implementation of the method.
A composite material part is constituted of fibers embedded in a resin matrix. The invention more particularly relates to the manufacturing of a composite material part from a preform of pre-impregnated fibers subjected to a polymerization cycle during which the fiber preform is compressed and subjected to a rise in temperature.
For certain material composite parts, the preform is obtained by stacking, on a convex tool, layers of pre-impregnated fibers on top of one another, the fibers being placed according to orientations, predetermined as a function of the mechanical characteristics which are sought. According to a mode of operation, each layer is constituted of juxtaposed fiber strips, the fibers being oriented along one direction for each strip. Generally, the fibers are constituted of several filaments and the thickness of the strip can vary as a function of the weight per unit area, and more particularly the number of filaments per fiber.
When the laying surface of the strips is developable and/or presents a non-pronounced radius of curvature, the strip is in contact with the laying surface over its entire width and entire length. In this case, the filaments of the strips all have almost the same tension.
This is very different when the laying surface is not developable. This is particularly the case of the composite material walls having a dual curvature as the nose fuselage of an aircraft or parts having variable thicknesses such as a longeron of an aircraft wing which has a section, variable over its entire length.
In the case of a non-developable surface, there is a difference of length between a first trajectory imposed by the surface on the filaments located in the area of the edges of the strip and a second trajectory imposed by the surface on the filaments located in the area of the other edge of the strip. Insofar as the filaments of the strip are quasi-inextensible, the filaments arranged near one of the edges will be tensioned whereas the filaments arranged in the area of the other edge will be compressed and will thus undulate.
There is also non-uniformity of the tensions of the filaments when strips having a high weight per unit area, and thus a great thickness, are laid over a small radius of curvature on the order of 10 to 30 mm, for example. Indeed, the filaments arranged in the area of the inner radius are compressed and tend to undulate.
The non-uniformity of the tensions of the filaments trapped in the resin matrix after polymerization causes a decrease of the mechanical characteristics of the part being manufactured. Indeed, the progressive mechanical loading of such a part causes a traction force to be exerted only on the filaments tensioned at first, then by increasing the load on the filaments somewhat less tensioned, and so forth, until the filaments the least tensioned, the most tensioned reaching their resistance limit to traction and being ready to break.
There are solutions to limit the risks of undulation, but none is entirely satisfactory.
A first solution includes laying the strips with much control, avoiding any play and overlapping of the strips. Indeed, the compressed filaments have a tendency to slacken in the overlapping and play zones, creating undulations therein. In the absence of such zones, even the compressed filaments tend to keep their positions. Even if this solution makes it possible to limit the risks of undulation, it does not solve the problem of non-uniformity of the tensions of the filaments and causes a significant decrease in productivity.
In the case of a non-developable surface, another solution includes deviating from the rule of orientation of the fibers imposed by the research department by favoring laying trajectories that are closer to the geodesic trajectories imposed by the laying surface. This solution causes the mechanical characteristics of the part to be reduced. In addition, it does not overcome the undulation risks in the case of a laying in the area of a strongly pronounced radius of curvature.
In the case of a non-developable surface, another solution includes reducing the width of the strip so as to limit the length difference between the trajectory followed by the filaments laid in the area of a first edge of the strip and the trajectory followed by the filaments laid in the area of the other edge of the strip. This solution is not satisfactory as it causes the number of strips to be increased, and thus the productivity to be significantly lowered. According to another limitation, it does not solve the problems related to laying a strip in the area of a very pronounced radius of curvature.
In the case of a very pronounced radius of curvature, a solution includes limiting the thickness of the strip (low weight per unit area). This solution is not satisfactory as it causes the number of layers to be laid to be increased and thus reduces productivity significantly. In addition, it does not solve the problems related to laying a strip on a non-developable surface.
Consequently, the methods of the prior art do not provide optimum results in terms of quality and productivity.
Therefore, the present invention aims at overcoming the drawbacks of the prior art.